A known drill adapter of this kind includes a forward part, to which the drill bit is attached, and a rearward part, said parts being rotatably joined at an inclined abutment surface common to both parts, so that when the forward adapter part is rotated relative to the rearward part said part is swung outwardly and angled obliquely to the geometric longitudinal axis of the drilling rod.
Another known device for eccentric compressed-air drilling purposes includes a so-called off-center drill bit which comprises a guide, an especially designed pilot drill, and a reamer arranged therebetween. The eccentric function of this device is achieved through the reamer, which by rotating the same through 180.degree. can be moved radially outwards and inwards relative to the pilot drill located in the drill hole and in front of the reamer.